The present invention relates to novel polymeric materials and is particularly concerned with rigid polymeric materials having a cellular core of a fiber-reinforced skin and a polymer film coating on the surface of the structure.
Rapid advances in polymer chemistry in recent years has led to an ever increasing use of synthetic materials for fabricating numerous types of structures which have heretofore been prepared from wood, metal, glass and like conventional materials. Reinforced polymers now, however, are used in a wide variety of structural applications. Indeed, laminates having a cellular core and outer layers of reinforcing sheets of materials adhesively bonded to the cellular core have gained considerable commercial attention because of their strength and weight characteristics.
As is readily appreciated in the art, the structural laminates which are fabricated by adhesively bonding various ply of laminae have strength characteristics which are limited to a considerable extent by the quality of the bond between the core and the skin. These materials mave shown a tendency over a period of time to delaminate when subjected to a great deal of flexural stress, thus shortening their useful life.
Attempts have been made to form structural laminates by foaming an expandable resin composition between sheets of reinforcing fibers such as glass fiber cloth or mats.
More recently, a method and apparatus has been developed for producing a fiber-reinforced foam article in which there are randomly oriented staple fiber filaments distributed throughout the foam in a predetermined concentration. When this fiber-reinforced foam is produced between woven fabrics, such as fiberglass fabric, substantial improvement in the mechanical strength of the foam results due to the "anchoring" of the fabric into the foam article by the randomly oriented fiber filaments. As a consequence thereof, this structural material or panel has gained wide acceptance because of its light weight and high strength. Indeed, the material has great utility in the production of light strong floors and partitions and especially in the construction of trailer truck bodies and the like.
In producing such fiber-reinforced foam structures, however, it is often highly desirable to provide the structural foam article with an exterior surface layer of film material, such as an acrylic plastic film material, to provide the article with a smooth and aesthetically pleasing surface. It would be particularly advantageous, when foaming the resin composition between layers of woven fabric, to be able also to place the plastic Film material in the mold so that a structural reinforced foam which has an attractive exterior finish is produced in a single step.
One area of difficulty associated with forming a foam article having an exterior layer of plastic film material and a layer of fiber-reinforcing sheet material is in providing an adequate bond between the exterior facing sheet and the foam. Another problem encountered is a blistering that occurs on the facing sheet. Indeed, blisters which develop on the facing sheet during the foaming step must then be sanded and the article resurfaced or repaired, otherwise the resultant foam article must be scraped. The need for resurfacing or the scraping of the article both are highly uneconomical. Consequently, there is a great commercial interest in developing a technique for foaming an expandable plastic composition between fiber-reinforcing skin material and facing sheets such that blistering of the facing sheet is substantially minimized if not totally avoided.